It is well known that the purity of diphenols is of paramount importance regarding the quality of polymers which are prepared therefrom. Isomeric diphenols that often accompany the p,p'-diphenols are often deleterious since they do not participate as well in polymerization processes. It is thus desirable and important to obtain the p,p'-diphenols in their highest purity in order to secure the quality of the polymers which are prepared therefrom. Since isomeric diphenols always accompany the desired p,p'-diphenols, purification of the crude reaction products is always necessary.
In the past, purification of crude mixtures of p,p'-diphenols was most often affected by the use of organic solvents such as benzene, methylene chloride or toluene. The use of these and similar solvents results in costly purification due to both the cost of solvents and inefficiency of the method. Another purification method is set forth in U.S. Pat. No. 3,919,330. This method involves dissolving crude 2,2-bis(4-hydroxyphenyl)propane in ethylene glycol and then adding a certain amount of water whereby the 2,2-bis(4-hydroxyphenyl)propane is precipitated and is then recovered. However, this method requires the use of large amounts of ethylene glycol and the necessity of an additional step of separating the water from the aqueous mother liquor in order to recover and reuse the anhydrous glycol.